standard illuminant C

[4.9.3.2] A representation of daylight having a correlated color temperature of approximately 6800 K. It is defined by its relative spectral power distribution over the range from 320 to 770 nm.*

^{* Rea MS (ed.). The IESNA Lighting Handbook, 9th ed. New York: Illuminating Engineering Society of North America; 2000. Figure 4-3.}